OGI is usually realized using a light emitter and a receiver. The light emitter commonly takes the form of a light emitting diode (LED), and the receiver takes the form of a photo diode (PD), to define a LED-PD combination.
In order to emit the requisite amount of light for detection by the PD, the LED is commonly made of material such as Ge, GaAs, InAs. In the most common form of light generation, photons are emitted by the recombination of electron-hole pairs. The amount of energy required to excite the electron and the amount of energy emitted in the form of light depends on the band gap of the material, which is related to the atomic structure of the material.
Atoms comprise a nucleus of protons and usually neutrons, with electrons occupying discrete energy levels or shells around the nucleus. When multiple atoms or molecules combine into a solid the resultant structure defines substantially continuous energy bands. These energy bands are however continuously varying due to varying energy distributions of electrons in the bands. Between the energy bands are regions where no electrons vibrate. Thus, in order for an electron to be excited to the next level, energy, e.g., heat has to be imparted on the electron to create an electron-hole pair in which the electron moves to a higher energy level. When an electron drops back to a lower energy level to recombine with a hole, the energy is emitted as light or sound. The minimal-energy state in the conduction band, and the maximal-energy state in the valence band, are each characterized by a certain k-vector in the Brillouin zone. If the k-vectors are the same, it is called a “direct gap”. If they are different, it is called an “indirect gap”.
FIG. 1 shows an energy diagram depicting energy vs. K norm curves for various materials. The low energy (non conductive) band, known as the valence band, is indicated by curve 100, which depicts the maximal energy state in the valence band. In order for an electron to be excited into the conduction band (band 102 for InAs; band 104 for GaAs; band 106 for Ge; band 108 for Si), which depict the minimal energy state in the conduction band of each of these materials, energy has to be imparted on the electron. Since the electrons can be excited to higher levels than the lowest energy level in the conduction band, excitation energies and subsequent recombination energies may be several times the band gap energy.
As is depicted in FIG. 1, the bands of the various materials have different widths. Also, in FIG. 1 all of the materials are semiconductor materials and the valence band is shown to be separated from each conduction band by a band gap, which can be thought of as continuously varying in width and position. (In the case of metals the valence band would overlap the conduction band, providing for free electron movement, however in the case of a semiconductor material, as shown in FIG. 1, a band gap exists between the valence band and the conduction band).In practice, in a lattice structure of atoms or molecules, electrons move not only in two dimensions as shown in FIG. 1. Instead, electron movement is in three dimensions and may be depicted by a wave vector (k) that has both magnitude and direction. This impacts the ability of electrons of a particular material to be excited to the conduction band. For example, in the case of GaAs (direct band gap material) the valence band is depicted as being directly below the bottom of the conduction band (i.e. the direction of the wave vector is in the same direction in the valence band to that in the conduction band) therefore but no change in momentum is required to excite and electron to the conduction band. Thus GaAs is called a direct band gap material. This is depicted in FIG. 2, which shows Energy vs. crystal momentum for a semiconductor with a direct band gap. In such a material an electron can shift from the lowest-energy state in the conduction band to the highest-energy state in the valence band without a change in crystal momentum. Depicted is a transition in which a photon excites an electron from the valence band to the conduction band.In contrast, in some other solids, such a Si the top of the valence band is not directly below the bottom of the conduction band (i.e. the direction of the wave vector is different in the valence band to that in the conduction band). Thus, not only is an addition of energy required but also a change in momentum in exciting an electron from the valence band to the conduction band in silicon. This is depicted in FIG. 3, which shows Energy vs. crystal momentum for a semiconductor with an indirect band gap, showing that an electron cannot shift between the lowest-energy state in the conduction band to the highest-energy state in the valence band without a change in momentum. Here, almost all of the energy comes from a photon (vertical arrow 300), while almost all of the momentum comes from a phonon (horizontal arrow 302).
Silicon is an example of an indirect band gap material and thus requires a momentum change for exciting an electron to the conduction band.
Interactions among electrons, holes, phonons, photons, and other particles are required to satisfy conservation of energy and crystal momentum (i.e., conservation of total k-vector). A photon with an energy near a semiconductor band gap has almost zero momentum. An important process is called radiative recombination, where an electron in the conduction band recombines with a hole in the valence band, releasing the full excess energy as a photon. This process is possible in a direct band gap semiconductor if the electron is near the bottom of the conduction band and the hole is near the top of the valence band (as is usually the case). However this process is not possible in an indirect band gap material, because conservation of crystal momentum would be violated. For radiative recombination to occur in an indirect band gap material, the process must also involve the absorption or emission of a phonon, where the phonon momentum equals the difference between the electron and hole momentum, or the energy difference can be achieved by a crystallographic defect, which performs essentially the same role. The involvement of the phonon makes this process much less likely to occur in a given span of time, which is why radiative recombination is far slower in indirect band gap materials than direct band gap ones. This is also why light-emitting and laser diodes are almost always made of direct band gap materials, and not indirect band gap material like silicon.The fact that radiative recombination is slow in indirect band gap materials also means that, under most circumstances, radiative recombinations will be a small proportion of total recombinations, with most recombinations being non-radiative, taking place at point defects or at grain boundaries. However, if the excited electrons are prevented from reaching these recombination places where they can combine in a non-radiative recombination, they have no choice but to eventually fall back into the valence band by radiative recombination, which can be enhanced by creating a dislocation loop in the material.Thus, while indirect band gap materials like Si can theoretically be excited to create electron-hole pairs, and the electrons can theoretically recombine in electron-hole pairs, emitting the energy as light, no practical Si LED has been developed because of the indirect band gap nature of silicon discussed above.The present invention proposes a practical avalanche LED (ALED) and photo diode (PD) pair made of silicon in which the ALED/PD combination is configured to arrange avalanche breakdown junctions of the ALED and PD in close proximity to each other and with different spacings to accommodate different wavelength energies.